The couple hasn't said whether or not they signed a prenup, but because of Zuckerberg's timing and where he lives that may not matter.
The reason: There are eight "community property states" and California just happens to be one of them.
According to Legalzoom, in a community property state like California, "spouses own- and owe- everything equally, regardless of who earns or spends the income." However, this only applies to assets acquired during the marriage. If the couple eventually splits, all the profits Zuckerberg made during the IPO would remain his.